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Geochimica et Cosmochimica Acta 71 (2007) 3979–3989

Interpreting the Ca isotope record of marine biogenic carbonates

Neil G. Sime a, Christina L. De La Rocha b,*, Edward T. Tipper c,Aradhna Tripati a, Albert Galy a, Michael J. Bickle a

a Department of Earth Sciences, University of Cambridge, UKb Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse, 27568 Bremerhaven, Germanyc Laboratoire de Geochimie-Cosmochimie, Institut de Physique du Globe de Paris-Universite Paris 7, France

Received 9 October 2006; accepted in revised form 11 June 2007; available online 23 June 2007

Abstract

An 18 million year record of the Ca isotopic composition (d44/42Ca) of planktonic foraminiferans from ODP site 925, in theAtlantic, on the Ceara Rise, provides the opportunity for critical analysis of Ca isotope-based reconstructions of the Ca cycle.d44/42Ca in this record averages +0.37 ± 0.05 (1r SD) and ranges from +0.21‰ to +0.52‰. The record is a good match topreviously published Neogene Ca isotope records based on foraminiferans, but is not similar to the record based on bulk car-bonates, which has values that are as much as 0.25‰ lower. Bulk carbonate and planktonic foraminiferans from core topsdiffer slightly in their d44/42Ca (i.e., by 0.06 ± 0.06‰ (n = 5)), while the difference between bulk carbonate and foraminiferanvalues further back in time is markedly larger, leaving open the question of the cause of the difference. Modeling the global Cacycle from downcore variations in d44/42Ca by assuming fixed values for the isotopic composition of weathering inputs(d44/42Caw) and for isotope fractionation associated with the production of carbonate sediments (Dsed) results in unrealisticallylarge variations in the total mass of Ca2+ in the oceans over the Neogene. Alternatively, variations of ±0.05‰ in the Ca iso-tope composition of weathering inputs or in the extent of fractionation of Ca isotopes during calcareous sediment formationcould entirely account for variations in the Ca isotopic composition of marine carbonates. Ca isotope fractionation duringcontinental weathering, such as has been recently observed, could easily result in variations in d44/42Caw of a few tenths ofpermil. Likewise a difference in the fractionation factors associated with aragonite versus calcite formation could drive shiftsin Dsed of tenths of permil with shifts in the relative output of calcite and aragonite from the ocean. Until better constraints onvariations in d44/42Caw and Dsed have been established, modeling the Ca2+ content of seawater from Ca isotope curves shouldbe approached cautiously.� 2007 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

The nature and significance of the mechanisms whichcause changes in global climate on long time scales, suchas transitions from greenhouse to icehouse modes, arepoorly understood, although there is general agreementthat the negative feedback between global temperature,weathering rates, and atmospheric CO2 has acted to

0016-7037/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.gca.2007.06.009

* Corresponding author. Fax: +49 (0) 471 4831 1923.E-mail address: Christina.De.La.Rocha@awi.de (C.L. De La

Rocha).

moderate global climate over most of Earth history (e.g.,Berner and Kothavala, 2001). Part of the problem inunderstanding the controls on past climate is the difficultyof reconstructing the key parameters (e.g., atmosphericCO2 concentrations and seawater pH and alkalinity) neces-sary to describe the long-term carbon cycle. It has beenproposed that the calcium isotopic composition of marinebiogenic carbonates can be used to reconstruct pastchanges in fluxes of Ca into and out of the oceans and pastchanges in atmospheric concentrations of CO2 (De La Ro-cha and De Paolo, 2000; De Paolo, 2004; Fantle and DePaolo, 2005; Heuser et al., 2005). Continental weatheringis the predominant source of calcium ions (Ca2+) to seawater

3980 N.G. Sime et al. / Geochimica et Cosmochimica Acta 71 (2007) 3979–3989

and as such, shifts in the Ca2+ content or isotopic compo-sition of ocean water over time hold information aboutweathering rates. In addition, in a surface ocean saturatedwith respect to carbonate minerals, the Ca isotopic compo-sition of Ca2+ should vary directly with pCO2 (De La Ro-cha and De Paolo, 2000).

There are now a number of published records of the Caisotopic composition of marine sediments (De La Rochaand De Paolo, 2000; Schmitt et al., 2003a; Gussone et al.,2004; Fantle and De Paolo, 2005; Heuser et al., 2005; Hip-pler et al., 2006), making it pertinent to critically reviewthe nature of Ca isotope variations and the methods bywhich they are interpreted. We present a new high-precisionrecord of the Ca isotopic composition of foraminiferans,and by extension, seawater, for the last 18 m.y., and com-pare it to other published long-term records over this inter-val. The relative significance of potential controls on the Caisotope composition of seawater, including shifts in boththe fluxes and isotopic composition of Ca into and out ofthe ocean, are evaluated.

2. MATERIALS AND METHODS

2.1. Separation and cleaning of foraminiferans and bulk

carbonates

Samples were selected from every �0.5 m.y. down corefrom ODP cores 925 A (4�12.2490N, 43�29.3340W, 3042mbsl) and 925 B (4�12.2480N, 43�29.3490W, 3041 mbsl),on the Ceara Rise, to span the range from 0 to 18 Ma.For each Ca isotope analysis, 10–60 morphologically-simi-lar adult specimens of a given species of planktonicforaminiferan (Globigerinoides trilobus s.l., Globigerinoides

sacculifer, and Orbulina universa), showing no obvioussigns of dissolution or recrystallization, were picked fromthe 300 to 350 lm size fraction. Picked samples werecrushed and cleaned following a method developed forMg/Ca measurements (Barker et al., 2003): clays were re-moved via ultrasonication, organic matter was removedvia oxidation with a solution of 1% H2O2 buffered with so-dium hydroxide, coarse-grained silicates were removedusing a fine brush under a microscope, and adsorbed con-taminants were removed via leaching and ultrasonication in0.001 M HNO3.

Samples of bulk carbonate were also taken from boxcore tops in the Atlantic and Indian Oceans and were pre-pared via dissolution of dried bulk sediment in 0.075 MHNO3. This weak strength of acid was used to minimizethe leaching of Ca from clays and non-carbonate particlespresent in the sediments. Undissolved particles were re-moved through dilution, centrifugation and further settlingovernight, followed by removal of the supernatant.

2.2. Purification of Ca for the measurement of Ca isotopes

Separated and cleaned samples of bulk carbonate andforaminiferans were dissolved in 1.2 M HCl and thenloaded onto quartz columns containing 1.8 ml of AG-50W-X8 ion exchange resin (Bio-Rad). Columns had beenpre-cleaned with 6 M HCl, rinsed with water, and pre-con-

ditioned with 1.2 M HCl. The Ca fraction eluted in the sec-ond fraction of 1.2 M HCl passed through the columns,and was collected, dried down, and re-dissolved in 0.3 MHNO3 for mass spectrometry. Procedural blanks were be-low 3 ng Ca, 5 · 10�3 % of the smallest sample analyzed.

2.3. Mass spectrometry

Ca isotope natural abundances were measured on a NuInstruments MC-ICP-MS following Halicz et al. (1999),using a standard-sample–standard-bracketing technique.Samples were introduced into the Ar plasma, operating atan RF power of 1300 W, via an Aridus desolvating nebu-lizer. For each measurement, 3 blocks of 200-s integrationswere collected, bracketed by four blocks of the SRM 915acalcium standard. Potential interference from Sr2+ wasmonitored at mass 43.5 and by measuring both the44Ca/42Ca and the 43Ca/42Ca ratio to ensure mass-depen-dent behavior. Samples determined to have a higher Sr toCa molar ratio than 4 · 10�5 or to deviate from the massfractionation line by more than 0.15‰ with respect tod43/42Ca were reprocessed through the column chemistry.

Owing to interferences at mass 40 from argon, 40Cacould not be measured. Data are presented instead in per-mil relative to the 44Ca to 42Ca ratio as d44/42Ca:

d44=42Ca ¼ Rsam

Rstd

� 1

� �� 103 ð1Þ

where Rsam and Rstd are the 44Ca to 42Ca ratio of a sampleand standard (NIST SRM 915a), respectively.

Over the duration of this study, the 1r standard devia-tions of measurements of the NIST SRM 915a standardand Specpure Ca solutions were ±0.035‰ (n = 93) and0.045‰ (n = 29), respectively, measured against NISTSRM 915a. The 1r standard deviation of six complete pro-cedural repeats on a synthetic standard comprising Ca fromNIST SRM 915a mixed with Na, Sr, K, Mn, and Mg inproportions equivalent to those in foraminiferal calcite,was 0.045‰.

Analyses of chemically purified samples were repeatedbetween 2 and 6 times. The root-mean standard deviationof the sets of repeats was 0.043‰ for G. sacculifer, 0.036‰for G. trilobus, and 0.030‰ for O. universa. When fittingcurves, we have conservatively assumed that the 1r preci-sion on multiple analyses of a single sample is 0.045‰ (thelargest of all our standard deviation uncertainties) andthat the 1r standard error on an average of a set of differ-ent samples is 0.045/(n�0.5), where n is the number of dif-ferent samples.

A large portion of the Ca isotope data published is re-ported with respect to 44Ca/40Ca (i.e., as d44/40Ca) insteadof versus 44Ca/42Ca. Multiplication of d44/40Ca values by0.4763 converts them to d44/42Ca, calculated from therelationship based on mi, the exact masses of theisotopes,

d44=42Ca ¼ d44=40Ca� 1

m42

� 1

m44

� �=

1

m40

� 1

m44

� �; ð2Þ

provided that the samples have been normalized to thesame standard.

Table 1The Ca isotopic composition of G. trilobus from ODP site 925

Depth (mbsf) Age (Ma) d44/42Ca (‰)a n

1.83 0.02 0.31 ± 0.04 416.04 0.45 0.39 ± 0.02 332.03 1.01 0.35 ± 0.03 345.93 1.50 0.45 ± 0.04 460.29 2.01 0.44 ± 0.03 474.28 2.49 0.40 ± 0.03 289.34 2.98 0.42 ± 0.04 4

103.93 3.52 0.40 ± 0.04 3115.63 3.97 0.44 ± 0.03 3128.14 4.48 0.52 ± 0.02 3141.77 5.00 0.37 ± 0.03 5152.18 5.50 0.42 ± 0.02 3161.30 5.96 0.39 ± 0.03 6176.90 6.51 0.37 ± 0.04 3192.39 6.98 0.42 ± 0.06 3200.69 7.50 0.37 ± 0.05 6208.94 7.99 0.33 ± 0.04 6226.24 8.51 0.34 ± 0.04 3234.57 9.02 0.37 ± 0.05 3240.72 9.50 0.31 ± 0.05 3247.64 9.95 0.41 ± 0.03 3261.89 10.59 0.39 ± 0.02 3269.67 11.11 0.39 ± 0.03 3276.31 11.49 0.36 ± 0.05 4282.22 12.02 0.34 ± 0.03 2289.66 12.47 0.33 ± 0.04 3298.07 13.00 0.37 ± 0.01 3304.74 13.55 0.30 ± 0.04 4315.93 13.98 0.33 ± 0.04 3314.69 14.14 0.27 ± 0.03 3323.36 14.56 0.26 ± 0.01 3342.84 15.51 0.25 ± 0.03 3357.85 16.15 0.30 ± 0.02 3369.39 16.52 0.23 ± 0.05 4381.69 17.06 0.26 ± 0.03 3411.03 18.11 0.43 ± 0.03 3

a Errors given are the standard deviation of n replicate analyses.

Interpreting Ca isotope records 3981

2.4. Determination of sample ages

An astronomically calibrated time scale for the CearaRise sediments has been developed by Bickert et al.(1997), Shackleton and Crowhurst (1997), Tiedemann andFranz (1997), and Shackleton et al. (1999). Sample agesin the present study were assigned using a revised versionof the time scale based on a new set of orbital solutions(Laskar et al., 2004).

2.5. Curve fits and smoothing

A smoothed curve has been fit to the foraminiferald44/42Ca records in order to remove from the Ca isotopecurve variability occurring at wavelengths shorter thanthe �0.75 m.y. residence time of Ca2+ in the ocean (as cal-culated from its modern concentration and flux estimates inMilliman, 1993). The slope of the Ca isotope variation withtime was calculated from the resulting curve for use in cal-culations of the Ca2+ content of the ocean.

There are many different ways to fit a curve throughdata. The approach taken here was to represent the Caisotope data as a Fourier series. To do this, the linear gra-dient (defined by the first and last data points in the timeseries) was first removed. Following standard procedure(Press et al., 1988), distortion of the curve induced bythe restriction of the sampling to a finite interval was sup-pressed by zero padding 35 points, spaced at 0.5 m.y.intervals, before and after the first and last data points.The Fourier coefficients were then calculated by singularvalue decomposition of the over-determined set ofequations.

The data were smoothed to remove high frequencywavelengths (which are more likely noise in the data ratherthan real variations in the isotopic signal). This was done byreducing the amplitude of the terms in the Fourier seriesthrough a diffusive smoothing term, e�p2n2b. In this term,n is the nth Fourier term and b is chosen so that an anomalywith the duration of the diffusion time constant, tc, is re-duced in amplitude by 1/e. A diffusion time equivalent to2.5 m.y. was used although this is longer than the time con-stant for changes in the Ca isotopic composition of theoceans (c.f. Richter and Turekian, 1993). When consider-ably more Ca isotope data points are available, more con-fidence in the higher frequency variations in the curve willbe possible and then it will be appropriate to choose asmoothing term closer to the residence time of Ca in theoceans.

Due to the small signal to uncertainty ratio, it is impor-tant to assess the uncertainties in the curve fit to the Ca iso-tope data. A Monte Carlo approach has been used here tobest estimate the uncertainty in the curve fitting. The calcu-lation was made using the standard error for d44/42Ca ateach time point (i.e., 0.045(n�0.5) where n is the numberof different species analyzed). The results of this error anal-ysis should be treated with some caution where the data arepadded to remove aliasing and smoothed to remove highfrequency noise. The Monte Carlo treatment illustrateshow sensitive the fit is to the estimated uncertainties inthe data.

3. RESULTS

3.1. Downcore variations in d44/42Ca of planktonic

foraminiferans

The d44/42Ca of 74 planktonic foraminiferan samples (36of G. trilobus, 24 of G. sacculifer, and 14 of O. universa) cov-ering last 18 m.y. averages +0.37 ± 0.05‰ (the error givenhere and hereafter as 1r SD unless otherwise noted) andranges from +0.21‰ to +0.52‰ (Tables 1–3). Over the14 sample interval for which there are data for all threespecies (1.50–7.99 Ma), the averages are +0.41 ± 0.04‰(G. trilobus), +0.40 ± 0.04‰ (G. sacculifer), and +0.36 ±0.03‰ (O. universa). The means of G. trilobus and G. saccu-

lifer cannot be statistically distinguished (Student’s t-test,a = 0.05, p = 0.42), although they both differ from that ofO. universa (Student’s t-test, a = 0.05, p < 0.01). There isno consistent offset between the data for each species fromeach particular sample for which data at the same depth areavailable from multiple species. Because of this and becauseof the previously noted lack of difference between species in

Table 2The Ca isotopic composition of G. sacculifer from ODP site 925

Depth (mbsf) Age (Ma) d44/42Ca (‰)a n

1.83 0.02 0.36 ± 0.03 316.04 0.45 0.35 ± 0.04 332.03 1.01 0.34 ± 0.03 345.93 1.50 0.43 ± 0.06 460.29 2.01 0.35 ± 0.04 374.28 2.49 0.37 ± 0.02 389.34 2.98 0.40 ± 0.02 3

103.93 3.52 0.46 ± 0.04 3115.63 3.97 0.40 ± 0.00 3128.14 4.48 0.42 ± 0.02 3141.77 5.01 0.41 ± 0.05 3152.18 5.50 0.32 ± 0.03 3161.30 5.96 0.36 ± 0.02 3176.90 6.51 0.46 ± 0.03 3192.39 6.98 0.35 ± 0.04 3200.69 7.50 0.35 ± 0.04 4208.94 7.99 0.34 ± 0.02 3226.24 8.51 0.41 ± 0.06 4234.57 9.02 0.41 ± 0.03 4240.72 9.50 0.39 ± 0.03 3247.64 9.95 0.40 ± 0.05 4261.89 10.59 0.40 ± 0.04 5289.66 12.47 0.38 ± 0.03 3314.69 14.14 0.21 ± 0.01 3

a Errors given are the standard deviation of n replicate analyses.

Table 3The Ca isotopic composition of O. universa from ODP site 925

Depth (mbsf) Age (Ma) d44/42Ca (‰)a n

45.93 1.50 0.32 ± 0.03 360.29 2.01 0.35 ± 0.03 374.28 2.49 0.32 ± 0.03 389.34 2.98 0.33 ± 0.03 3

103.93 3.52 0.36 ± 0.04 4115.63 3.97 0.37 ± 0.01 3128.14 4.48 0.41 ± 0.04 3141.77 5.01 0.35 ± 0.00 3152.18 5.50 0.37 ± 0.03 3161.30 5.96 0.39 ± 0.02 3176.90 6.51 0.34 ± 0.00 3192.39 6.98 0.37 ± 0.02 3200.69 7.50 0.39 ± 0.03 3208.94 7.99 0.37 ± 0.05 3

a Errors given are the standard deviation of n replicate analyses.

δ44/4

2 Ca

(‰)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

δ44/4

2 Ca

(‰)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Age (Ma)0510152025

δ44/4

2 Ca

(‰)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

δ44/4

2 Ca

(‰)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Fig. 1. (a) The d44/42Ca of planktonic foraminiferans from ODPsite 925 shown with error bars that are 1r standard deviation of 2–4 replicate measurements of each sample and the best-fit curvethrough the data filtered at a wavelength of 2.5 m.y. (solid line)and the 1-r uncertainty on the curve fit (dashed lines). Circlesrepresent G. trilobus, squares are O. universa, and triangles areG. sacculifer. (b) d44/42Ca from G. bulloides from ODP site 183, andG. ruber, Globigerinella, and G. trilobus from ODP site 144 (Heuseret al., 2005). The solid line in the second panel is the best-fit curvethrough the Heuser et al. (2005) data filtered at a wavelength of2.5 m.y. (c) The fitted curves from ODP site 925 (bold solid line)and from Heuser et al. (2005) (bold dashed line) and the 1-runcertainty on the curve fits (non-bold dashed lines). (d) Acompilation of the ODP site 925 data and the Heuser et al. (2005)data 18 Ma and older. The lines shown are the best fit curve filteredat a wavelength of 2.5 m.y. and the associated 1r standard errorinterval.

3982 N.G. Sime et al. / Geochimica et Cosmochimica Acta 71 (2007) 3979–3989

Ca isotopic fractionation in core-top specimens (Sime et al.,2005), the data from these three species were compiled intoa single record of the d44/42Ca of planktonic foraminiferansover time (Fig. 1a).

The Fourier fit through the data (excluding the 18 Masample), smoothed with a 2.5 m.y. time constant, showsthe major features of the d44/42Ca record (Fig. 1a). Movingforwards in time, between 17 and 10 Ma, values increasefrom +0.26 to +0.38‰. From 10 to 6 Ma, values remainaround +0.37‰. An increase to +0.40‰ by 4 Ma is fol-lowed by a decline to +0.35‰ by the present day.

Table 4The Ca isotopic composition of bulk carbonates versus foraminiferans from box core tops

Box core Location Water depth (mbsl) d44/42Cabulk (‰)a n d44/42Caforam (‰)b n

NEAP 5b 61�04.500N, 24�31.760W 1826 0.30 ± 0.03 3 0.30 1OMEX 16b 48�50.070N, 12�39.85085W 2335 0.26 ± 0.03 3 0.33 ± 0.17 6OMEX 9b 49�59.700N, 12�31.100W 2281 0.25 ± 0.07 3 0.33 ± 0.17 6WIND 22b 13�36.850S, 51�11.380E 3838 0.26 ± 0.02 3 0.40 ± 0.22 9WIND 33b 11�12.710S, 58�46.240E 3520 0.26 ± 0.02 3 0.28 ± 0.12 7

a Uncertainty is reported as the standard deviation of n replicate measurements of the same bulk carbonate sample.b Uncertainty is reported as the standard deviation of the n data points available (Sime et al., 2005) for planktonic foraminiferans of various

species in these box cores.

Interpreting Ca isotope records 3983

The 18 m.y. record presented here is comparable to the23.5 m.y. record of Heuser et al. (2005) in Fig. 1b. Thetwo data sets coincide, within error, and the 2.5 m.y.smoothed fits essentially overlap at the 1r level (Fig. 1c).The two records show a similar rise in values between 17and 10 Ma and they oscillate about similar baseline valuesbetween 10 and 4 Ma.

Because the general trends and absolute values are sim-ilar in the Heuser et al. (2005) data and the data presentedhere (Fig. 1c), the data from Heuser et al. (2005) have beenincluded to extend the record considered to 23.5 Ma(Fig. 1d). However, because the uncertainty (estimatedfrom the scatter about a smoothed fit with a 2.5 m.y. timeconstant) on the data set published by Heuser et al.(2005) is much larger than the one presented here (i.e.,0.08‰ versus 0.024‰), data from Heuser et al. (2005) havenot be included between 17.5 Ma and the present day. Theirinclusion increases the uncertainty on the curve fit whilemaking little difference to its shape and position.

3.2. d44/42Ca of bulk carbonates

The d44/42Ca of bulk carbonate samples from five boxcore tops in the Atlantic and Indian Oceans (Table 4) aver-ages +0.27 ± 0.02‰, ranging from +0.25 to +0.30‰. Thebulk carbonate d44/42Ca values run from being equal tothe average value for foraminiferans in those cores, to being0.14‰ more negative, for an average difference betweenthe foraminiferan and bulk carbonate in the box coresof 0.06 ± 0.06‰. The average bulk carbonate value of+0.27‰ is also 0.06‰ lower than the average value forplanktonic foraminiferans (+0.33‰) from the suite ofcore tops presented in Sime et al. (2005). Although themean difference between the bulk carbonate and foraminif-eran values in each core top is statistically significant(Student’s t-test, a = 0.05, p = 0.04), the difference betweenthe two values falls within the standard deviation of multi-ple analyses of the foraminiferan samples (Table 4). Giventhe small set of samples analyzed, is not possible to con-clude whether the differences are real or analytical.

4. DISCUSSION

4.1. Use of Ca isotopes of foraminiferans to track marine

d44/42Ca

Several studies have suggested that the Ca isotopic com-position of planktonic foraminiferans reliably tracks the Ca

isotopic composition of seawater and can be used to recon-struct Ca isotope records over time (De La Rocha and DePaolo, 2000; De Paolo, 2004; Fantle and De Paolo, 2005).Examination of 12 species of foraminiferan (including thethree species reported here) in box core tops suggested that,on the whole, modern planktonic foraminiferans producecalcium carbonate tests that have a d44/42Ca that is�0.65‰ relative to seawater, and that this Ca isotope frac-tionation is not significantly affected by temperature normeasurably different between species (Sime et al., 2005).A similarly negligible relationship between temperatureand Ca isotope fractionation has been observed in culturingstudies (�0.008‰ per �C for O. universa; Gussone et al.,2003) and four different species of planktonic foraminiferanfrom marine sediments have yielded similar Ca isotope re-cords over the last 20 m.y. (Heuser et al., 2005).

Although Ca isotopes in most species of foraminiferanappear to be, at most, slightly affected by temperature, astrong temperature dependence of Ca isotope fractionationhas been observed in G. sacculifer (Nagler et al., 2000; Gus-sone et al., 2004; Hippler et al., 2006). Despite this, resultsfrom G. sacculifer have been included in the downcorereconstructions of d44/42Ca presented here (Fig. 1). Thereare several observations supporting this. First, as pointedout above, at ODP site 925, the mean and standard devia-tion of the d44/42Ca of G. sacculifer are equal to those ofG. trilobus, a species for which a strong temperature depen-dence has never been reported (Heuser et al., 2005; Simeet al., 2005). Second, a large response of Ca isotopes to tem-perature has not been consistently observed for G. sacculif-

er; e.g., a study of Ca isotopes in foraminiferans from boxcore tops failed to observe a large or even significant rela-tionship between Ca isotopes and temperature in G. saccu-

lifer (Sime et al., 2005). Additionally, the d44/42Ca ofG. sacculifer over its 14 million years in the ODP site 925record ranges by only 0.25‰ (Table 2), nowhere near thelarge range expected if temperature was a significant driverof variations (e.g., the 0.7‰ range within 100 kyr ofHippler et al., 2006). Lastly, the similarity of the recordsof several different species of planktonic foraminiferan fromthe equatorial Atlantic (ODP Site 925), the equatorialPacific (ODP Sites 871 and 872; Heuser et al., 2005), andthe Indian sector of the Southern Ocean (ODP Site 1138;Heuser et al., 2005), as shown in Fig. 1, suggests that thereis a common driver influencing the Ca isotopes on long timescales. Given the equatorial to polar span in the locations ofthe sediment cores, it is improbable that the common driveris temperature.

3984 N.G. Sime et al. / Geochimica et Cosmochimica Acta 71 (2007) 3979–3989

4.2. Comparison with other records of the Ca isotopic

composition of seawater

The measured fractionation of �0.65‰ between seawa-ter and foraminiferal calcite (Sime et al., 2005) may be usedto convert foraminiferan d44/42Ca values to those of seawa-ter. The resulting seawater Ca isotope record may be com-pared to others that have been constructed (Fig. 2) frombulk carbonates (De La Rocha and De Paolo, 2000; Fantleand De Paolo, 2005) or marine authigenic apatites (Schmittet al., 2003a).

Although the foraminiferan and bulk carbonate recordsoverlap for the first couple of million years, after that thebulk carbonate values are generally more negative thanthe foraminiferan-based data (Fig. 2), at times by as muchas 0.2–0.3‰. The isotopic difference between bulk carbon-ate and seawater reported by Fantle and De Paolo (2005),�0.62‰, is within error of the �0.65‰ difference reportedbetween planktonic foraminiferans in sediment core topsand seawater (Sime et al., 2005). Since the bulk carbonatedata and the foraminiferan data were converted to seawatervalues using these nearly identical values, the divergences inthe two seawater curves back in time are driven almost so-lely by differences in the Ca isotopic composition of thebulk carbonates and the foraminiferans.

The first potential reason for this is that bulk carbonatesfrom the deep sea consist of both foraminiferans and thecalcium carbonate liths of coccolithophores. If these twomaterials, which make up a different proportion of the sed-iment at any given time and place, have differing isotopiccompositions, the bulk carbonate record could be offsetfrom and have different inflections than the record basedon foraminiferans. In support of this possibility are recentculturing experiments where the coccolithophore, Emiliania

huxleyi, produced calcium carbonate that was �0.15‰ low-er than that produced by the foraminiferan, O. universa

(Gussone et al., 2006). Likewise the modern bulk carbon-ates analyzed (which, based on their Sr/Ca ratio, wereroughly 50/50 coccolith/foraminiferan (Sime, 2006)) have

Age (Ma)0510152025

δ44/4

2 Ca

(‰)

0.7

0.8

0.9

1.0

1.1

1.2

Fig. 2. Estimates of the d44/42Ca of seawater from differentmaterials. Compared in this plot are values reconstructed fromthe foraminiferal data [filled symbols as in Fig. 1 but withdiamonds representing data from Heuser et al. (2005)] and2.5 m.y. smoothed best fit curve (solid line) from Fig. 1d, fromauthigenic apatite (grey circles; Schmitt et al., 2003b) and bulkcarbonates (open squares and dashed line; Fantle and De Paolo,2005).

a d44/42Ca that is an average of 0.06 ± 0.06‰ lower thanthe planktonic foraminiferans in them (Table 4). Althoughmore work needs to be done on isotopic fractionation byboth foraminiferans and coccolithophores to pin downthe exact values, the 0.15‰ difference in the fractionationfactors lends scope for slight differences in the Ca isotoperecords based on bulk carbonates and foraminiferans. Italso suggests that the �0.62‰ offset used by Fantle andDe Paolo (2005) may not be a negative enough value.

Because the difference between the bulk carbonate andforaminiferan data is sometimes greater than 0.15‰, andthe nannofossil oozes of Fantle and De Paolo (2005) con-tain a significant proportion of foraminiferans, it seemsunlikely that the differences between the two curves aredriven entirely by the presence of coccoliths in the bulkcarbonate samples. Given that the records diverge afteran initial few million years of agreement (Table 4 andFig. 2), diagenesis may be affecting bulk carbonate materi-als to a different extent than the foraminiferans. For exam-ple, the foraminiferans consist of hand-picked specimensshowing no signs of dissolution or recrystallizationwhereas, by the age of 10 Ma, the bulk carbonates havebeen about 40% recrystallized (Fantle and De Paolo,2005). Bulk carbonates might also contain authigenic apa-tites which have a slightly different d44/42Ca than carbon-ates, although the slightly higher apatite value wouldshift the bulk carbonate values towards the positive, thewrong direction to explain the difference between bulkand foraminiferal data sets. The leaching carried outduring the preparation of bulk carbonate samples maydissolve non-carbonate materials that contain Ca, suchas volcanic ash and clays, which do exist in some of thebulk carbonates in the downcore record (Fantle and DePaolo, 2005). These bulk carbonates, which are fromDSDP site 590B on Lord Howe Rise in the Pacific Ocean,might also contain variable quantities of aragonite (in theform of fragments of pteropods; c.f. Exon et al., 2004) whichhas a different Ca isotopic composition than calcite (Gussoneet al., 2003; Marriott et al., 2004; Gussone et al., 2005).

The reconstructed seawater d44/42Ca based on apatites(Schmitt et al., 2003a) do not help resolve the quandary be-tween the bulk carbonate and foraminiferan records as theapatite data consist of a limited number of analyses whichscatter variably about the foraminiferan and bulk carbon-ate data (Fig. 2). Systematic work on all three materials isneeded to identify which is the most appropriate for theconstruction of seawater Ca isotope records. In the mean-time, we propose that the foraminiferan-based data setsare the most reliable because the calcium source is knownand does not consist of variable proportions of differingmaterials. In addition, the extent of potential diageneticmodification is limited by the hand selection and chemicalcharacterization of the samples analyzed (Sime, 2006).

4.3. Inversion of the Ca isotope data

Shifts in the Ca isotopic composition of seawater mayresult from imbalance between the fluxes of calcium inand out of the ocean, from changes in the isotopic compo-sition of Ca input to the ocean, or variations in the average

Rel

ativ

e oc

ean

NC

a

0

1

2

3

4

Rel

ativ

e oc

ean

NC

a

0

1

2

3

4

Rel

ativ

e oc

ean

NC

a

0

1

2

3

4

Age (Ma)05101520

CC

D (

km)

3

4

5

6

Fig. 3. Estimates of the mass of Ca2+ in the oceans (relative to thepresent day total). (a) shows estimates for a fixed d44/42Caw of+0.30‰ (dashed line) and of +0.36‰ (solid line). (b) shows relativechanges in the mass of oceanic Ca2+ and the 1-r uncertainty basedon a fixed d44/42Caw of +0.335‰. (c) shows the curves ford44/42Caw = +0.335‰ for the ODP 925 data extended with theolder data from Heuser et al., 2005 (grey line), a 2.5 m.y. smoothedfit of the ODP 925 data alone (solid black line), and a 0.75 m.y.smoothed fit of the ODP 925 data alone (dashed black line). Forcomparison, (d) shows the average depth of the CCD in the globalocean (compiled using CCD data from Lyle (2003) and Thunelland Corliss (1986) and hypsographic data from Kennett, 1982).

Interpreting Ca isotope records 3985

Ca isotope fractionation factor between seawater and theCa exported from the ocean as carbonate sediments (DeLa Rocha and De Paolo, 2000; De Paolo, 2004; Fantleand De Paolo, 2005). This may be modeled by definingthe change in the amount of Ca in the ocean (NCa) overtime (t) as the difference between the flux of Ca2+ in dueto weathering inputs (Fw) and the flux of Ca2+ out in theform of calcareous sediments (Fsed):

dN Ca

dt¼ F w � F sed ð3Þ

This model can be expanded to include changes in the Caisotopic composition of seawater by multiplying the fluxterms by their isotopic composition:

dðN Cad44=42CaswÞdt

¼ F wd44=42Caw � F sedðd44=42Casw þ DsedÞ

ð4Þ

In these equations, t is time (forward), NCa is the number ofmoles of Ca2+ present in seawater, d44/42Casw is the isotopiccomposition of seawater, Fw is the total input flux of Ca2+

to the ocean (mainly river and mid-ocean ridge hydrother-mal fluxes) and d44/42Caw is its isotopic composition, andFsed is the output flux of Ca2+ from the ocean and Dsed

is its average isotopic difference from seawater. Eq. (4)is equivalent to the formulation used previously (De LaRocha and De Paolo, 2000; De Paolo, 2004; Fantle andDe Paolo, 2005).

Several of the terms in Eqs. (3) and (4) may be madenon-dimensional by dividing by Fsed to yield

sdN 0Ca

dtd44=42Casw þ N 0Ca

dd44=42Casw

dt

� �

¼ F 0wd44=42Caw � ðd44=42Casw þ DsedÞ ð5Þ

and

sdN 0Ca

dt¼ F 0w � 1 ð6Þ

where s is the residence time of Ca in the ocean (i.e.,NCa/Fsed) at the present day, N 0Ca is the dimensionless massof Ca in the oceans relative to today, and F 0w is the dimen-sionless input flux of Ca to the ocean relative to the outputflux which is held constant (dividing Eqs. (3) and (4) by Fw

instead of Fsed, and holding Fw constant, produces similarresults).

Eqs. (5) and (6) are solved by eliminating F 0w (or F 0sedÞand the resulting differential equation is solved numericallyusing a 4th order Runge-Kutta scheme with time varyingvalues of d44/42Ca and d44/42Ca/dt taken from the smoothedFourier fit to the Ca isotope data in Fig. 1d. Unlike De LaRocha and De Paolo (2000); De Paolo (2004); and Fantleand De Paolo (2005), we do not assume that the residencetime of Ca in seawater is constant.The equation is solvedforward in time and the initial value of N 0Ca is adjusted togive a present day value of unity with the present day valueof s of 0.75 m.y. The calculations are carried out inside thesame Monte Carlo routine used to estimate the sensitivityof the smoothed Fourier fit to analytical uncertainties inthe data. This provides an estimate of the cumulate uncer-tainties in the model results.

4.4. Models of the variation in seawater d44/42Ca over the last

24 million years

Variation in the mass of oceanic Ca2+ with time over thelast 24 m.y. (Fig. 3) has been calculated from the 2.5 m.y.

3986 N.G. Sime et al. / Geochimica et Cosmochimica Acta 71 (2007) 3979–3989

smoothed fit (Fig. 1d) and the solutions to Eqs. (5) and (6).This requires ascribing fixed values for d44/42Caw and Dsed.As a first pass, a Dsed of �0.665‰ has been used along with+0.30‰ as the Ca isotopic composition of Ca inputs to theocean (d44/42Caw). This results in an estimate of the Ca2+

content of the ocean at 24 Ma of one fifth of the modernday value (Fig. 3a). Using a slightly different (but equallyplausible) value for d44/42Caw of +0.36‰ produces, on theother hand, an estimate for 24 Ma of nearly 3 times thatof the modern value. As expected from the equations, asimilar sensitivity is seen if Dsed is varied over a comparablerange and d44/42Caw is held constant.

Sensitivity of the Ca2+ concentration reconstructions tothe value of these parameters has been observed in previousstudies (e.g., Fantle and De Paolo, 2005) and is really nosurprise. The basic model (Eq. (4)) ties the calculated massof the weathering and sedimentary fluxes (Fw and Fsed)explicitly to the values selected for d44/42Caw and Dsed. Itgoes without saying that the d44/42Caw and Dsed values usedin the modeling must accurately represent the average isoto-pic composition of the weathering and sedimentary fluxes.What is now also obvious (Fig. 3) is that these parametersmust be known quite precisely (to the hundredths of permil)in order to be of use in models that extend back over severalto tens of millions of years.

Is there enough information for the average values ofd44/42Caw and Dsed over the Neogene and Quaternary tobe precisely constrained? The case for d44/42Caw can betaken first. Given that the Ca isotopic composition of marinecarbonates appears to have been similar to the present dayduring the Phanerzoic (Farkas et al., 2006; Steuber andBuhl, 2006) and perhaps back to the Neoproterozoic(Kasemann et al., 2005), the long term average isotopiccomposition of Ca inputs must be equal to that of theoutputs. On this basis, d44/42Caw could be defined as thebaseline average value of deep sea sediments.

Producing a precise average for the average Ca isotopiccomposition of marine sediments back through time turnsout to be a difficult thing to do either accurately or pre-cisely. First there is the problem that one gets a differentaverage depending on whether foraminiferan or bulk car-bonate records are used. The mean value of foraminiferalcarbonate based on the data set presented here combinedwith that of Heuser et al. (2005); is +0.334 ± 0.007‰ (1rstandard error) versus the +0.300 ± 0.013‰ (1r SE) ofthe bulk carbonate record (De La Rocha and De Paolo,2000; Fantle and De Paolo, 2005) over a similar interval.While a difference on the order of 3 hundredths of a permilseems small, it is large enough to produce significantly dif-ferent results over a few million years using the model of theCa cycle in general use for the Ca isotope-based reconstruc-tions. It should also be pointed out that these averages arebased only on a handful of locations in the ocean, all ofwhich occur in the deep sea. A precise, long term averagevalue of the d44/42Ca of marine sediments, also requiresknowledge of the isotopic composition of shallow carbon-ates, as discussed below.

A different way to know the Ca isotopic composition ofthe weathering fluxes is to construct a budget based on therelative fluxes and absolute isotopic compositions of the dif-

ferent inputs of Ca2+ to the ocean. The two inputs contrib-uting the overwhelming majority of ‘‘new’’ Ca2+ to theocean are rivers and hydrothermal fluids (Milliman, 1993)and so the values of these two together should be quite closeto the long term average isotopic composition of carbonatesediments. The current best estimate of the modern d44/42Caof the flux-weighted riverine Ca2+ input is +0.48 ± 0.13‰(1r SE) (Tipper, 2006). Estimates of the d44/42Ca of midocean ridge hydrothermal input is +0.44 ± 0.05‰ (1r SE,n = 3) (Schmitt et al., 2003b). These values are roughly0.15‰ higher than the long term average value of forami-niferans and bulk carbonates. Either this indicates a systemthat is currently far from isotopic steady state, a conditionthat is unlikely given the isotopic similarity of the Phanero-zoic and Neoproterozoic carbonates, or it indicates thatglobal fluxes of Ca isotopes into the sea are inadequatelycharacterized.

The value of Dsed, the average difference between the cal-cium carbonate output from seawater and seawater itself, isalso not precisely constrained. Although the bulk carbonaterecord yields a satisfactory view of the Ca isotopic compo-sition of calcitic deep sea sediments, it lends no insight intothe isotopic composition of material buried on the shelves.The shelves, which contain abundant corals and other ara-gonitic materials and, unlike the deep sea, lie above the ara-gonite lysocline, should contain a reasonably highproportion of aragonite. Deep sea sediments should containmostly calcite. Both the biotic and the abiotic precipitationof aragonite discriminate against the heavier isotopes ofcalcium to a greater degree than does the biotic and abioticformation of calcite (Gussone et al., 2005; Bohm et al.,2006), resulting in the production of aragonite with a lowerd44/42Ca than calcite. Thus the global ocean average valuefor Dsed should be somewhat lower than the estimates of�0.62 to �0.65‰ based on foraminiferans (Sime et al.,2005) and deep sea bulk carbonates (Fantle and De Paolo,2005).

Although the exact values of d44/42Caw and Dsed are notyet exactly known, for purposes of illustration we have se-lected two reasonable values from the middle of the spec-trum and modeled from them the Ca2+ content ofseawater over the last 24 million years. Using a Dsed of�0.665‰ (i.e., slightly more negative than the fractionationassociated with biogenic calcite formation) and a d44/42Caw

of +0.335‰ yields a Ca2+ curve that begins and ends withthe value of the present day (Fig. 3b), although there is anear doubling of Ca2+ between about 18 and 14 Ma(Fig. 3b), after which Ca2+ fairly steadily drops towardsthe present day. This result is similar to the calculationsof Heuser et al. (2005) who also observe a factor of two in-crease at �14 Ma. But the results differ from those of Fan-tle and De Paolo (2005) who find a maximum in Ca2+

between 6 and 4 Ma and conclude that the amount ofCa2+ in the ocean has decreased by three quarters betweenthen and the present day. The different results of Fantle andDe Paolo (2005) stem from the differences between the bulkcarbonate and foraminiferan isotopic records discussedearlier.

While we do not argue that the record presented here(Fig. 3c) represents the true change in the Ca2+ content of

Age (Ma)05101520

Δ sed

(‰)

-1.0

-0.8

-0.6

-0.4

δ44

/42 C

a w (

‰)

0.0

0.2

0.4

0.6

Fig. 4. Variations in (a) d44/42Caw or (b) Dsed required to drive theshifts in seawater Ca isotopic composition implied by the forami-niferan d44/42Ca at ODP site 925, assuming no change in the massof Ca2+ in the ocean and a 0.75 m.y. residence time for Ca2+.

Interpreting Ca isotope records 3987

the ocean over this interval, it is one of the most muted,conservative curves that can be modeled from the Ca iso-tope record of Fig. 1 using fixed values for d44/42Caw andDsed. Such large changes in the oceanic mass of Ca2+ be-tween 14 Ma and the present day seem implausible. Theyalso conflict with other modeling estimates (e.g., Wilkinsonand Algeo, 1989; Holland, 2005) and with reconstructionsbased on marine evaporites (Horita et al., 2002) that sug-gest that the concentration of Ca2+ in seawater fluctuatedby less than 30% over this interval. Further, the shifts inthe average oceanic calcite compensation depth (CCD),assuming no change in the CaCO3 production rate, indicatethat the calcite saturation state of seawater increased be-tween 14 Ma and the present day after decreasing between24 and 15 Ma (Fig. 3d). Because the calcite saturation stateis controlled by the ion activity product of Ca2+ and CO3

2�,two parameters whose concentrations should increase whenweathering inputs exceed carbonate outputs, the CCDreconstructions and Ca isotope-based model results contra-dict each other.

The implausibly large fluctuations in the oceanic Ca2+

mass inferred from the modeling of the Ca isotope curve(Fig. 1d) using fixed values for d44/42Caw and Dsed, coupledwith the sensitivity of the calculations to these parameters,imply that the values of d44/42Caw and Dsed fluctuate overtime and contribute to the variations observed in seawaterd44/42Ca. There are additional reasons to suspect that boththe isotopic composition of the inputs and the seawater car-bonate fractionation factors vary over time.

The production, during weathering on the continents, ofsecondary Ca-containing materials appears to fractionateCa isotopes (Tipper et al., 2006), as does Ca-uptake by ter-restrial vegetation (Skulan et al., 1999; Schmitt et al., 2003a;Wiegand et al., 2005). The Ca isotope fractionation associ-ated with the formation of these materials is on the order oftenths of permil and occurs in the right direction to explainthe riverine d44/42Ca values which are unexpectedly higherthan those of carbonate rocks and sediments (Tipperet al., 2006). If the terrestrial reservoir of soil carbonates,calcareous cements, and biogenic materials, such as calciumoxalate phytoliths, is large and long lived enough, the vari-ations in the amount of weathered Ca sequestered into thisreservoir would result in variability in the d44/42Ca of Ca in-put to the ocean. Approximately two thirds of the continen-tal weathering flux is derived from carbonate weatheringwith the remainder being derived from silicate and evapo-rate minerals (Milliman, 1993). Subtle changes in the pro-portion of Ca derived from each rock type could furtherlead to subtle changes in the d44/42Ca of the weathering flux.

While it is currently possible to acknowledge thatd44/42Caw may vary sizably enough and for long enoughstretches of time to cause variation in seawater d44/42Ca,more work is needed to confirm or refute the idea. The0.4‰ range of variation observed so far in the d44/42Ca ofCa2+ of rivers (Zhu and MacDougall, 1998; Schmittet al., 2003b; Tipper et al., 2006) serves, at least, as a theo-retical upper limit to the variations possible in the d44/42Caof Ca2+ input to the sea, at least over geologically shorttime scales. Over time scales of several hundred thousandyears (i.e., those long enough to notably impact the Ca iso-

topic composition of seawater), the upper limit of d44/42Caw

variations will be undoubtedly smaller. How much smallercan only be said through further modeling work and/orextended characterization of the behavior of Ca isotopesduring terrestrial Ca cycling.

In terms of processes that would result in shifts in theglobal ocean average value of Dsed, studies of Ca isotopefractionation during abiogenic and biogenic precipitationof calcite and aragonite (Gussone et al., 2003; Marriottet al., 2004; Sime et al., 2005; Gussone et al., 2005; Bohmet al., 2006; Gussone et al., 2006) indicate that Ca isotopesare fractionated to a differing extent during the productionof these two minerals. Although there are not yet enoughdata to constrain the difference in the d44/42Ca of the vari-ous different types of carbonate sediments that dominatethe output of Ca from the sea, the implication of the differ-ent fractionation factors is that any process that alters theproportion of Ca exported as aragonite versus calcite maydrive a significant change in the global ocean value of Dsed.

Such processes include a shift in extent of carbonate depo-sition on shallow platforms versus in the deep sea (Milli-man, 1993), or a change in the Mg2+ and Ca2+

concentrations of seawater (Sandberg, 1983; Stanley andHardie, 1998).

An interesting exercise is to model variations ind44/42Caw or Dsed from Eq. (5) and the Ca isotope recordby holding the mass of Ca2+ in the oceans (N0Ca), and thusthe input and output fluxes of Ca (F 0CaÞ, constant. Thissheds light on the magnitude and duration of variationsin d44/42Caw or Dsed that could by themselves be responsiblefor the changes in the Ca isotopic composition of seawater.These curves are shown in Fig. 4. It appears that variations

3988 N.G. Sime et al. / Geochimica et Cosmochimica Acta 71 (2007) 3979–3989

in d44/42Caw or Dsed of ±0.05‰ sustained over a few millionyears each are sufficient to explain the entirety of the vari-ations seawater Ca isotope curve over the last 24 Ma. Giventhis, it seems inappropriate for the Ca2+ content of seawa-ter to be modeled from Ca isotope curves using fixed valuesfor d44/42Caw and Dsed.

5. CONCLUSIONS

Analyses of foraminiferal d44/42Ca ratios provide a con-sistent and sufficiently precise record to constrain variationsin past seawater d44/42Ca values over the last 24 m.y. Overthis time period, seawater d44/42Ca exhibits a range of atleast 0.1‰. Modeling the variations in seawater d44/42Caas due only to changes in the fluxes of Ca into and out ofthe ocean requires unreasonably large variations the totalamount of Ca2+ present in the ocean. The alternative expla-nation, that the variations in seawater d44/42Ca reflect smallvariations (±0.05‰) in the mean isotopic composition ofthe inputs or in the average fractionation factor for carbon-ate sediment production, is more plausible. It seems proba-ble that changes in the relative proportions of differentcarbonate types contributing to the sediment mass or shiftsin the relative importance of shallow platform versus deepsea loci for carbonate burial, may have changed the averageoffset between marine carbonates and seawater by the fewtenths of permil needed to explain much of the variabilityin seawater d44/42Ca values. It is not yet known if Ca iso-tope fractionation during weathering and secondary car-bonate formation can drive shifts in riverine d44/42Ca thatare large enough and sustained for intervals long enoughto have a visible effect.

The amount of Ca2+ in the oceans has certainly variedover time. However, resolving the Ca isotopic variation asso-ciated with this variation from those resulting from changesin d44/42Caw and Dsed requires much better records of thecomposition of carbonate sediments, and better quantifica-tion of isotope fractionation factors associated with the inputand output fluxes of Ca2+ to and from the oceans.

ACKNOWLEDGMENT

This work was supported by NERC (Grant NER/M/S/2001/00032 to CDLR and studentship NER/S/A/2001/05963 toN.G.S.). We are grateful to N. Shackleton and S. Crowhurst forproviding updated age models for cores 925a and 925b, and toH. Elderfield, M.Greaves, L. Booth, M. Hall, C. Lear, and I.McCave for samples, advice, or assistance. The comments of M.Fantle and two further anonymous reviewers greatly improved thismanuscript. This work is dedicated, fondly and with respect, toN.J.S.

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Associate editor: James Farquhar